This application claims the priority of Japanese Patent Application No. 10-275122 filed on Sep. 29, 1998, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a grating for measuring a pattern projection, which is used in a moire apparatus or the like.
2. Description of the Prior Art
Moire apparatus have conventionally been known as apparatus for easily capturing three-dimensional information of an object to be measured in a short period of time. The moire apparatus include those of a grating irradiation type and a grating projection type. The latter would have a higher degree of flexibility in measuring the object to be measured since they need no reference grating as in the former.
The grating projection type moire apparatus comprises a projection optical system and an observation optical system which have their respective optical axes parallel to each other; and is configured such that the projection optical system projects an image of a projection grating onto the object to be measured, the deformed grating image formed on the object is focused onto an observation reference grating by the observation optical system, and the resulting moire fringe is observed.
From the viewpoint of enhancing the accuracy in observing the moire fringe, it is desirable that a grating having a density distribution as closer as possible to a sine wave be employed as the above-mentioned projection grating and observation grating.
For realizing this feature, a pattern projection measurement grating having a pseudo sine wave density distribution constituted by a black and white binary pattern is considered. In this specification, xe2x80x9cwhitexe2x80x9d in xe2x80x9cblack and white binary patternxe2x80x9d refers to xe2x80x9ctransparentxe2x80x9d whereas xe2x80x9cblackxe2x80x9d refers to xe2x80x9copaque.xe2x80x9d
Possible examples of the method of making a pattern projection measurement grating having a pseudo sine wave density distribution such as that mentioned above include a method comprising the steps of placing a platemaking screen over a sine wave density distribution pattern so as to change the latter to a black and white binary pattern in which areas of the black and white dots vary, and then reducing thus obtained pattern to an actual grating size; and a method in which, though a black and white binary pattern is formed by black and white dots, the dots are formed randomly, while random numbers are generated in a computer so as to form a function for carrying out dot distribution control, and are drawn such that the dot distribution is formed like a sine wave.
In any of the cases where the above-mentioned methods are employed, it is hard to make a pattern projection measurement grating having a preferable pseudo sine wave density distribution.
Namely, in the above-mentioned methods, black parts get darker (opaque parts become darker) since the influence of diffraction and the like cannot be taken into consideration, whereas white parts get brighter due to the minimum black dot dimensions that can be made, whereby the resulting pattern would basically look like an enlarged silver halide photograph.
In view of such circumstances, it is an object of the present invention to provide a pattern projection measurement grating having a pseudo sine wave density distribution constituted by a black and white binary pattern, in which the pseudo sine wave density distribution can be made as a density distribution close to a sine wave.
The projection measurement grating in accordance with the present invention is a pattern projection measurement grating having a pseudo sine wave density distribution constituted by a black and white binary pattern,
wherein predetermined sections dividing each cycle of a total amplitude of a sine wave are provided, each section being divided into black and white regions according to a ratio between the area of the sine wave and the area of a rectangular wave having the same total amplitude as that of the sine wave in this section, so as to constitute the black and white binary pattern.
Preferably, in this case, the black and white binary pattern is formed with a density which cannot be resolved by a resolving power of a projection measurement lens.
Also, it is preferred that the black and white regions be divided by forming a black or white band-like region which passes near a center of gravity of a differential region between the rectangular wave and the sine wave.
Preferably, the black and white binary pattern is set to a pattern symmetrical about a position at xcfx80, in which black and white are inverted about positions at xcfx80/2 and 3xcfx80/2.
Further, it is preferred that the black and white regions be divided like a band, whereas the white region has a width corrected, according to the degree of diffraction of light transmitted through the white region, to a width greater than the width set by the above-mentioned ratio.